The plasma-electrolytic oxidation of surfaces of lightweight metals is a known process. It produces predominantly hard, ceramic layers which offer corrosion protection and wear protection. A prerequisite for plasma-electrolytic oxidation is the formation of an oxide layer (dielectric) in an electrolyte. Maintenance of a current can thus lead to an increase in voltage and discharges. In this way, the surface of lightweight metal parts is converted into a ceramic matrix. This usually requires an electric potential of at least 250 V, which brings about a spark discharge at the surfaces of the parts; local plasma formation occurs. The layers are formed by means of microdischarges which melt the substrate material and reaction products of the electrolyte with the lightweight metal and sinter to form a crystalline ceramic. Alkali silicate or phosphate solutions are predominantly used as electrolyte.
The production of coatings on lightweight metal components by plasma-electrolytic oxidation is described, for example, in C. Blawert et al., Advanced Engineering Materials 2006, 8, No. 6, pages 511 to 533, which is hereby incorporated by reference.
Particles have also for some time been incorporated in the layers. For example, Srinivasan et al., Surface Engineering 2010, Vol. 26, No. 5, pages 367 to 370, describe the coating of magnesium alloys of the AM50 type in alkaline phosphate solutions with addition of TiO2 sol. These particles are incorporated and (partially) crystalline layers are formed.